1. Field of the Invention
This invention relates to signal detector circuits and, more particularly, to the detection of loop current in a telephone subscriber loop occurring in response to the removal of the handset in response to telephone ringing, i.e., ring-trip detection.
2. Description of the Prior Art
It has become common to supply telephone subscribers with telephone service over pair gain systems. A pair gain system is a signal multiplexing system which supplies more than one telephone subscriber channel over a single pair of twisted wires. Such pair gain systems are very useful in situations where the cost of providing metallic pairs for each subscriber is prohibitive. They are also useful in situations where it is desirable to defer the laying of new cable until sufficient demand develops or until unknown demand levels are ascertained. A final application of pair gain systems is to provide temporary service to a particular location such as, for example, the site of a political or athletic event.
In such pair gain systems, it is not possible to transmit ringing signals over the twisted pair since ringing signals must be provided for a plurality of different subscribers at the remote end of the pair gain system. In this situation, ringing signals are generated at the remote location and selectively applied to the particular subscribers by control signals sent over the pair gain system. It is desirable in such a situation that the ringing signals be provided by means of apparatus which is small in size and economical and consumes little power since such apparatus must be located in a remote location.
One of the ancilliary features required with such remote ringing supplies is the ability to trip the ringing signal, i.e., terminate the ringing signal, when the called subscriber goes off-hook. This off-hook condition can be detected from the current which flows through the subscriber telephone set when it is placed in the off-hook condition. Considerable difficulties are encountered, however, in detecting this relatively small direct current flow in the presence of extremely large alternating current ringing signals.
The problem, then, is to detect ring-trip currents in the presence of the ringing signal by means of circuitry which is small, inexpensive and rugged and thus suitable for use in the remote location on a per subscriber basis.